Mercury
Mercury is the smallest planet in our solar system and the closest to the Sun. With virtually no atmosphere to retain heat, it experiences the greatest temperature variations of any planet, from scorching hot to freezing cold.
Equatorial Diameter
4,879 km
Temperature
473°C (day) / -183°C (night)
Mass
$3.3 \times 10^{23} \text{ kg}$
Surface Gravity
$3.7 \text{ m/s}^2$
Rotation Period
59 Earth Days
Solar Day
176 Earth Days
Planetary Structure
Mercury's extremely high density suggests it has a very large, iron-rich core, which may be molten. This core has a higher iron content than that of any other major planet in the Solar System. Above the core sits a mantle of solid silicate rock and a crust of silicate rock at the surface. The crust is heavily cratered, similar in appearance to the Moon.
Scientific Discoveries
- Orbits the Sun at a mean distance of 60.3 million km.
- Completes an orbit around the Sun in just 88 Earth days, the fastest of all planets.
- The surface features numerous narrow ridges, which may have formed as Mercury's core and mantle cooled and contracted after the crust had solidified.
- Despite its proximity to the Sun, water ice exists in permanently shadowed craters at its poles.
- From Earth, Mercury appears as a morning or evening star, but it can be hard to see. It is best observed twice a year: in spring at dusk and in autumn before dawn.
- It has a very thin atmosphere, called an exosphere, composed mostly of oxygen, sodium, and hydrogen.